“Southern steelhead are crucial indicators of watershed health,” Jacobson said.

She said restoring the “aquatic highways” the fish use to reach their spawning habitats will also bring benefits for people, including safeguarding sources of clean drinking water.

“I am hopeful for steelhead recovery,” Jacobson said. California’s classification of the population as endangered, she said, will help advance a state conservation plan and add urgency to the work of removing barriers in rivers.

The steelhead trout that remain in Southern California face other threats, including warmer waters and more intense droughts and wildfires as a result of climate change.

“These are populations that are experiencing the warmest conditions, really on the leading edge of climate change effects. And then you layer on top of that just how densely populated Southern California is,” said Andrew Rypel, a professor of fish ecology and director of UC Davis’ Center for Watershed Sciences. “All of these steelhead streams in Southern California are extremely impacted.”

He said that with so many factors weighing against the steelhead trout, the additional protections could make a significant difference.

“It’s like the most challenging fish conservation issue I can imagine,” Rypel said. “How do you manage a whole landscape for fish conservation in the middle of one of the biggest urban areas in the world? It’s very challenging.”

This population of steelhead, he said, is effectively “up against the clock.”

Removal of barriers to spawning areas is key, he said.

“It’s a really cool fish. It’s a Southern California fish, and it’s up to the people of that region to watch out for it and to ensure that future generations are going to be able to watch this cool fish and protect it — and by way of doing that, protect the ecosystem.”

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